Lasers: Protecting the Starship

by Paul Gilster on April 12, 2012

Interesting new ideas about asteroid deflection are coming out of the University of Strathclyde (Glasgow), involving the use of lasers in coordinated satellite swarms to change an asteroid’s trajectory. This is useful work in its own right, but I also want to mention it in terms of a broader topic we often return to: How to deal with the harmful effects of dust and interstellar gas on a fast-moving starship. That’s a discussion that has played out many a time over the past eight years in these pages, but it’s as lively a topic as ever, and one on which we’re going to need a lot more information before true interstellar missions can take place.

Lasers and the Asteroid

But let’s set the stage at Strathclyde for a moment. The idea here is to send small satellites capable of formation flying with the asteroid, all of them firing their lasers at close range. The university’s Massimiliano Vasile, who is leading this work, says that the challenge of lasers in space is to combine high power, high efficiency and high beam quality simultaneously. He adds:

“The additional problem with asteroid deflection is that when the laser begins to break down the surface of the object, the plume of gas and debris impinges the spacecraft and contaminates the laser. However, our laboratory tests have proven that the level of contamination is less than expected and the laser could continue to function for longer than anticipated.”

Vasile believes using a flotilla of small but agile spacecraft, each with a highly efficient laser, is more feasible than trying to deflect an asteroid with a single, large spacecraft carrying a much larger laser. One benefit is that the system is scalable — add as many spacecraft as needed for the job at hand. The other is that you have the redundancy afforded by multiple laser platforms. The Strathclyde work is also investigating whether a similar system could be used to remove space debris by de-orbiting problematic objects to avoid potential collisions.

Erosion Shields on the Starship?

If lasers can be used to alter an asteroid’s trajectory, we need to consider their uses in clearing out the space ahead of futuristic space probes. That the interstellar medium itself was going to be a problem became apparent as researchers began to study starship deceleration concepts in the early 1970s. Get your vehicle moving in the range of 0.3 c and any grain of carbonaceous dust a tenth of a micron in diameter it encounters carries a relative kinetic energy of 37,500,000 GeV, according to Dana Andrews (Andrews Space) in a 2004 paper. How that kinetic energy is dealt with is clearly a major issue.

By the late 1970s, aluminum and then graphite had been considered as possible erosion shields, with the preference going to graphite, but in 1978 Anthony Martin reviewed the literature and suggested a beryllium payload shield be deployed on the Project Daedalus probe, which would be moving at .12 c. It would be quite a large object, some 9 millimeters thick and 32 meters in radius, and even it didn’t completely solve the problem, for Daedalus would, upon arrival, be moving into a still denser gas and dust environment around Barnard’s Star. Daedalus designer Alan Bond suggested additional shielding in the form of a cloud of dust deployed from the probe, which would vaporize larger particles before they could damage the vehicle.

Image: Diagram of the Project Daedalus probe, developed by the British Interplanetary Society in the 1970s. Note the beryllium shield at upper left. Credit: Adrian Mann.

Clearing Out the Path

We’re still not through, though. What about particles larger than dust grains, up to hailstones in size? We are now talking about collisions that would be catastrophic, and must turn from passive to active measures to tackle the problem. Gregory Matloff and Eugene Mallove have suggested using a light or X-ray laser or a neutral particle beam firing ahead of the ship to deflect any objects detected in its forward-pointing radar. The Project Icarus team has looked at creating a bumper out of graphene, as discussed in this blog entry, and coupling it with a laser defense:

What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts.

The notion seems an adaptation of Conley Powell’s 1975 work on shields that move ahead of the ship, trapping ionized material on impact within a magnetic field. The earlier Daedalus researchers found that Powell’s ideas resulted in less erosion than other methods then being studied. This is an interesting shield, one placed perhaps 100 kilometers ahead of the spacecraft. Moreover, it is not passive but can signal the vehicle when grains have passed through it without being ionized:

This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology.

All of which takes me back to the University of Strathclyde work on laser deflection, and makes me wonder whether laser technologies first deployed against asteroids in our Solar System may one day be used to protect our interstellar voyagers.

Anthony Martin’s paper is “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (Journal of the British Interplanetary Society, 1978): S116–S121. Alan Bond discusses in-system shielding in “Project Daedalus: Target System Encounter Protection,” S123–S125 in the same publication. The Dana Andrews paper is “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.

We could take an asteroid, one half of it would be solid, the other half a hollowed-out volume for the crew and cargo, and the solid half would be the shield. It would be a worldship, and some people were recommending that idea anyway.
I know, it would be hard to accelerate such a large mass.
I wonder if the particles would be striking the ship with enough energy to induce thermonuclear fusion?
I mentioned negative mass earlier. It’s an impractical solution because we don’t know where to get any. But I wondered if there are other reasons it’s impractical?

Why do the erosion shields appear flat? Why not use the cow-catcher/deflector approach and make they highly conical. Then the particles would be deflected and a large part of their energy would not have to be absorbed?

I always thought that a very large, cool and very low density gas cloud traveling with the vessel could take care of random molecules and objects up to the size of dust ( 100 nmeter). it could be provided by pushing out , say , 1 gram slugs of ice at 5 to 50 meters per second forward in the same direction as the ship. These then slowly evaporated or can be evaporated with a low intensity laser to make a cloud of slowly expanding cool gas. a significant gas cloud on the order of 1000 km deep is possible, though at a low density.
the frequency of larger particles in inner stellar space is pretty low , given the low extinction co0efficient for light in deep space. a thin shield ( . 1mm thick) in front of the ship will vaporize anything up to a millimeter or more. Anything big will be detectable by radar or its interaction with the (putative) gas cloud . It is really big you will just have to deflect it with a projectile ( shoot the dang thing) or steer the vessel to miss it.

Some concerns I have with a laser based deflection systems, is 1) detecting something that may only be big as a pinhead from 100Km distance 2) actually hitting said particle from 100Km, you only get one shot within that range, it’s safer if it was detected and shot at 1000Km distance, giving a better chance if you miss the first time.
I’ve always liked the idea of a plasma shield, something like what the m2p2 (Mini-Magnetospheric Plasma Propulsion). Of course you would need to have decent plasma densities to actually work.

A shield, whether dust or solid, that is disconnected from the ship is only useful during inertial flight. Since it is likely that the ship will spend the majority of the voyage accelerating or decelerating, the shield has to be part of the ship.

Most of the flight will likely be inertial or low thrust. Here is why: if you accelerate to the half way point then decellerate the resto fo the way, the average speed of the voyage is half of the top speed. thus you use 4 times more energy with little reduction in total mission length. A practical technology will be under full acceleration phase for perhaps 10% or less of the journey.

Now that we’re studying the nuts & bolts of ship protection,
we see our intrepid starship generating quite a forward signature:
1. Occasional high-power laser bursts
(admittedly diverging after the focal point)
2. High-energy particle collisions making light, UV, x-Rays, as well as slower charged particles.
3. The declerating rocket burn aimed, of course, at the destination. A laser-imploding fusion rocket should have a very strong signature.

This renders absurd any notion of aliens sneaking up on us for clandestine spying, which itself is far more difficult now than even 50 years ago.

stephen: “I mentioned negative mass earlier. It’s an impractical solution because we don’t know where to get any. But I wondered if there are other reasons it’s impractical?”

That there is no reason to believe that it exists.

kittlej: “A practical technology will be under full acceleration phase for perhaps 10% or less of the journey.”

Either a very long journey or a non-existent propulsion system.

Mark: “…the particles would be deflected…”

At those high velocities there will unfortunately be no deflections, just destructive impacts.

Greg: “Some concerns I have with a laser based deflection systems…”

Not only true, it’s even worse. The radar cross-section of a “hail stone” is impossibly small at any distance at which a reactive defense can hope to act. There is also no way to locate the projectile to allow the laser to aim at it after it’s been detected, and the vast majority will be false positives. There is also the small matter that a continuous wave radar is a photon rocket that will act in opposition to the main engines, while a pulsed radar will (statistically speaking) miss almost all threats.

“Why do the erosion shields appear flat? Why not use the cow-catcher/deflector approach and make they highly conical. Then the particles would be deflected and a large part of their energy would not have to be absorbed? ”

The relative speeds are such that the particles are more accurately described as really heavy nuclear radiation, rather than grains of sand. The moment they contact anything solid you get an extremely violent explosion.

I would think the best approach would be a mini-magnetospheric shield for dealing with charged particles, combined with a solar sail well ahead of it to ensure that any solid objects *become* charged particles.

I have sometimes wondered; At star ship speeds, how much of a magnetic field would you need to convert solid obstacles into plasma via dielectric breakdown under induced voltage?

I presented a very low mass solution to the dust problem at the 100 Year Starship Symposium in a talk titled “Dust Grain Damage to Interstellar Vehicles and Lightsails”.
An earlier published paper contains most of the important physics:
Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail”, Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.

Interstellar Bill: This blog has discussed non-rocket based methods of slowing down, so aliens won’t necessarily use #3. As for #1 and #2, would we even recognize such signatures as distinct from background noise, especially galactic cosmic rays?

Or if you really want to be speculative, could such forward signatures account for observations like the Oh-My-God particle?

With using lasers/particle beams we have the advantage of as the spacecraft gains speed it adds it velocity to the energy of the beam, so you could start off with a low frequency light which would then be shifted towards the high energy part of the spectrum as the craft gains speed. Also any gas ahead that is pushed to oneside will start to act as a ionised bow shock plasma limiting the movement of particles moving through it.
I thought about the idea of using the final stage of the Daedalus turned around (when still attached to the second stage) which would fire every now and then, the stream of particles from the exhaust would clear a space for sure, waste fuel though.

This is a very interesting topic that has something to compare to on Earth: anti-missile systems onboards military vessels. Since it is rather difficult to intercept an incoming anti-ship missile – which is small, fast and somewhat “intelligent” -, weaponry on ships is often organized using a multi-layer concept. Different types of weapons and ammunition are used at successive stages of approach. At closer range, very fast automtic cannons – Gatling or Vulcan guns – are employed, but at larger distances, shrapnel-producing projectiles, anti-missile missiles and so on might be used as well. Indeed, space is different but perhaps some ides about the general concept of naval multi-layered weapons systems could be borrowed.

Ron S I did not say we had a practical propulsion system yet, just that the energy required to accelerate half way and flip over then decelerate the other half is MUCH higher than a mission with shorter acceleration periods. This factor becomes less important if the propulsion system can do its job in 10% or less of the total mission time. Personally I do not yet see many prospects to get above 0.01C with the technologies we presently have or can extrapolate with simple engineering. But when I first started doing DNA Sequencing in 1980 a good day had us decoding maybe 200 base pairs in a week, now we can easily do a whole human genome ( 3 billion basepairs) in a week. or even a day. -AND at that time we did not see even how the physics, biochemistry or information processing of such a sequencing system might work. Whole new biochemistry was developed to do this.
Ditto for micro processors
Ditto for internet connectivity
ditto for manufacturing ( with still a lot more to go , compared to building things one at a time by hand using home made tools)
I read and comment on this blog because it is possible that when the ideas start to flow, human ingenuity can achieve amazing ends

I have sometimes wondered; At star ship speeds, how much of a magnetic field would you need to convert solid obstacles into plasma via dielectric breakdown under induced voltage?

I have wondered this, too, recently on this blog. It is a showdown between atomic electric fields (~10^10 V/m), and and the Lorentz force (~10^9 V/m per Tesla at v = c, newtonian). Thus, the field needed to ionize gas on impact is on the order of 10-100 Tesla, a little on the high side but not totally out of the question for a low cross section device. For a sail, of course, there is not much hope.

jkittle, I did say “non-existent”, which is simply a fact. While I think you’re being too optimistic about how propulsion technology will progress, I am not saying we cannot get there. But until then high-acceleration, high-velocity travel is speculative. I prefer to extrapolate to something we can plan around.

Ron S I did not say we had a practical propulsion system yet, just that the energy required to accelerate half way and flip over then decelerate the other half is MUCH higher than a mission with shorter acceleration periods.

That depends on what you hold constant. We all know that the energy is proportional to the square of velocity, so it is not surprising that you need MUCH less energy if you accelerate for shorter times at the same acceleration. You also arrive centuries later, though.

If, on the other hand, you want to specify a certain travel time, there is not that much difference in energy between short and high acceleration vs. long and low. Short and high is somewhat faster for the same energy, but not greatly so.

Since lower acceleration requires less mass, both structural and in the engine, the high premium on mass will likely push the optimum towards low acceleration for long times, i.e. towards the “accelerate halfway/decelerate the other half” profile.

Although your correct in certain aspects, let’s take a Rail-gun that shoots a 1 ounce non-conductive bullet at 2 miles per second, how would a ship detect and then respond to that? Then put it in context of a star-ship, you have the cross section of a pea and its coming at you in multiples of thousands of miles of seconds. A missile you can track by it’s heat signature besides radar, and luckily its relatively slow so response times are easily attainable.

I still think that a high gauss (Tesla level) magnetic field expanded out by a few miles could do some damage to a dust particle that has some metallic components, at interstellar velocities I would think the magnetic field would appear as a pulse to the dust particle and the current generated would be so rapid that it would turn the particle into a small plasma burst. Of course this hypothesis needs to be tested and of course if the particle was completed made of a non-conductive material and it did not hold any kind of charge, there would be no point in it. I believe though that dust particles would hold some small charge.

Why do the erosion shields appear flat? Why not use the cow-catcher/deflector approach and make they highly conical. Then the particles would be deflected and a large part of their energy would not have to be absorbed? ”

Brett:
The relative speeds are such that the particles are more accurately described as really heavy nuclear radiation, rather than grains of sand. The moment they contact anything solid you get an extremely violent explosion.

Even so, a conical shape would deflect the momentum of those explosions, which would be directed orthogonal to the surface, at worst. It would likely also reduce the amount of material emerging on the other side of the shield. Most importantly, it would reduce the impact energy area density on the shield, which would help in keeping it cool and intact. I strongly suspect that shields will be conical (or slanted in some other way) for one or more of these reasons.

“I have wondered this, too, recently on this blog. It is a showdown between atomic electric fields (~10^10 V/m), and and the Lorentz force (~10^9 V/m per Tesla at v = c, newtonian). Thus, the field needed to ionize gas on impact is on the order of 10-100 Tesla, a little on the high side but not totally out of the question for a low cross section device. For a sail, of course, there is not much hope.”

For actual particles, as opposed to isolated neutral atoms, I’d expect the relevant field to be on the order of 10^8 V/m, the dielectric strength of ruby Mica. Vacuum UV would probably assure the presence of charge carriers to help initiate an avalanche discharge.

I would like to point out that there is a fair abundance of the H3+ ion in interstellar media and in molecular gas clouds. It plays a large role in cooling these clouds and perhaps stars would not condense with out its influence. It is a triangle shaped molecular ion and has lots of asymmetric vibration modes so it can radiate thermal energy efficiently. Of course there are molecular ions that contain deuterium as well , ( H2D+) which are even more asymmetric. . so… if you want interact with the interstellar medium using a magnetic field the ions are out there. (http://www.redorbit.com/news/space/1112513541/study-finds-way-to-observe-ion-responsible-for-early-star-formation/)

Also note that the nuclear spins of H2 neutral hydrogen molecule) would line up parallel or anti parallel to the field and thus H2 is slightly paramagnetic. H2 molecules in an magnetic field can , on average move along the magnetic field lines or if they are already in motion , be deflected as they cross the magnetic field. I wonder if this is a big enough effect to transfer any meaningful momentum to a ship? FYI these nuclear spins are responsible for the signal generated in a medical magnetic resonance imager…

Off topic – but not really. I see the US is building a new “Zumwalt” class of naval destroyers, essentially a late 19th century hull design, fossil fueled, but with some modern electronic “bells and whistles”. Initially 32 were planned, then 24 … after the cost (including R&D) ballooned to $7 billion each, now they will have to settle for just three ships. In this engineering and economic environment, it is hard to worry overmuch about how to shield craft traveling at significant fractions of c.

IMO, despite the rather credible Orion concept, I doubt that H sapiens will ever manage to build anything faster than 300 km/sec, and the shielding for that velocity is a doodle.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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